FIELD OF THE INVENTION

The invention refers to a system and a method for signal processing in a communication system (e.g. an optical communication system) .

BACKGROUND OF THE INVENTION

Driven by growing demand on transmission capacities, sophisticated modulation formats with improved spectral efficiency are becoming the established technology-of-choice for commer ^¬ cial usage in optical communication systems. Relatively robust and price effective on-off keying, widely applied for data rates of up to 10 Gbit/s, is step by step replaced by transmission formats modulated in phase and pola ^¬ rization, enabling further increase of data rates to 40 Gbit/s, 100 Gbit/s and higher. Hereto, combination of polarization multiplexing and coherent signal detection, namely "coherently-detected polarization- multiplexed quadrature phase shift keying" (CP-QPSK) , has been identified as a modulation format of choice for next generation high capacity transmission. The main CP-QPSK drivers are that it doubles the spectral ef ^¬ ficiency and the total capacity, it is part of the 100G stan ^¬ dardization according to the Optical Internetworking Forum (OIF), and all major system houses and component suppliers are currently working on CP-QPSK solutions.

On system level, it is of interest to reuse already imple ^¬ mented concepts when introducing new modulation formats. This applies also for optical pump sources that are currently used in context with different applications.

As an example of applications for optical pump sources it is worthwhile to mention high power pump lasers for Raman amplifiers, which enable a further increase of span length or idler channels which guarantee optimum EDFA operation in Dense Wavelength Division Multiplexing (DWDM) systems with limited total channel count as well as link stabilization in submarine transmission systems, or dynamically controlled Continuous Wave (CW) channels, which allow for network stabi- lization in case of sudden power transients (e.g. due to fi ^¬ ber cut) .

In all of the aforementioned applications, the power level of the pump signal is usually higher than the average power lev ^¬ el of the in-service data signals. In order to prevent pola- rization dependent gain ripple or signal distortions due to FWM interactions or polarization-dependent Raman gain, an un- polarized CW light or a filtered ASE source is conventionally employed .

All these techniques are well known in the art and used in current Dense Wavelength Division Multiplexing (DWDM) systems with conventional modulation formats as, for example, On Off Keying (OOK) , Optical Duo Binary (ODB) , Differential Phase Shift Keying (DPSK) or Differential Quadrature Phase Shift Keying (DQPSK) . However, much more than other modulation formats, modulation formats based on polarization multiplexing are sensitive to random or deterministic polarization rotations induced by cross-polarization modulation (XPolM) between the optical signals propagating along a transmission link.

In particular, a critical role is played by the spectral re ^¬ gion in which such polarization rotations fall. Accordingly, the following classification can be introduced:

• Slow polarization rotations, whose spectral components fall within the adaptive compensation speed of the receiver

(< 100 kHz) . These rotations are typically caused by mechani ^¬ cal and thermal stress effects cumulating during fiber propa ^¬ gation and the receiver is designed to fully compensate for them. · Fast polarization rotations, whose spectral components exceed the adaptive compensation speed of the receiver but fall within its electrical bandwidth (between 100 kHz and the polarization tracking cut-off frequency) . These polarization rotations result from XPolM effects cumulating during fiber propagation and the adaptive algorithms within the receiver are not fast enough to compensate for them.

• Ultra-fast polarization rotations, whose spectral compo ^¬ nents fall outside the electrical bandwidth of the receiver (above the polarization tracking cut-off frequency) . These polarization rotations also result from XPolM effects cumu ^¬ lating during fiber propagation, but they are so fast that they are rejected by the electrical filter of the receiver and therefore must not be compensated for by the adaptive al ^¬ gorithms of the receiver itself. Fig. 1 is a schematic representation of the application of a conventional depolarization technique. In particular, Fig 1 shows a combination of two delayed fractions of a single CW light 11, the first fraction over a Variable Optical Attenua ^¬ tor 14 and the second fraction over an optical fiber 13 characterized by a fiber length L > L _coh , where L _coh is the cohe- rence length.

Fig. 2 is a schematic representation of the application of a conventional depolarization technique. In particular, Fig 2 shows the combination 22 of two orthogonally polarized CW signals 23 and 26 from a single laser source 21. It shows al ^¬ so the Polarization Beam Splitter PBS 24, which splits the single laser source 21 in two orthogonally polarized CW sig ^¬ nals 23 and 26, and a Polarization Beam Combiner PBC 25, which combines the two orthogonally polarized CW signals 23 and 26. Fig. 3 is a representation 31 of the degradation of the Bit Error Ratio BER 32 of a 40G CP-QPSK optical signal due to a depolarized CW channel. In particular, Fig 3 shows the Bit Error Ratio BER 32 of a 40G CP-QPSK optical signal versus the difference between the power of the Continuous Wave CW pump signal and the power of CP-QPSK optical signal for three dif ^¬ ferent cases:

- a combination 34 of two delayed fractions of a single CW light, the CW signal being located 50GHz away from the CP-QPSK optical signal; - a combination 35 of two orthogonally polarized CW sig ^¬ nals and from a single laser source, the CW signal being located 50GHz away from the CP-QPSK optical signal;

- a combination 36 of two orthogonally polarized CW sig ^¬ nals and from a single laser source, the CW signal being located 2.6THz away from the CP-QPSK optical signal. In the example, the CW pump and 40G CP-QPSK co-propagate over a 700 km fiber link. The CW power level has been continuously increased relatively to the data signal. As shown in Fig. 3, significant bit-error ratio degradations in the CP-QPSK sig- nal can be observed starting at power differences of 1 dB . When the CW signal is a direct neighbor of the CP-QPSK data signal FEC threshold of 10 ^~3 is violated when exceeding 5 dB pump-data power difference. Furthermore, significant distor ^¬ tions have been detected even when the CW signal is located 2.5THz away from the CP-QPSK channel.

The example represented in Fig. 3 clearly shows that a con ^¬ ventional depolarized CW signal causes significant penalties for polarization multiplexed data signals over a very wide spectral region, whose width depends on the power of the CW signal itself. For this reason, due to high power difference, similar distortions can be expected also from co-propagating Raman pumps, thus strongly limiting its application in optical transmission systems with polarization-multiplexed chan ^¬ nels. As an alternative, the use of filtered Amplified Spontaneous Emission (ASE) light has been investigated. In contrary to intentionally depolarized CW signals characterized by fast but deterministic polarization rotations, ASE light is com ^¬ pletely depolarized, therefore polarization rotations are completely randomized and all polarization states are represented in it with the same probability.

Fig. 4 is a representation 41 of the degradation of the Bit Error Ratio BER 42 of a 40G CP-QPSK optical signal due to a Amplified Spontaneous Emission (ASE) source. In particular, Fig 4 shows the Bit Error Ratio BER 42 of a 40G CP-QPSK opti ^¬ cal signal versus the difference between the power of the Am- plified Spontaneous Emission (ASE) source and the power of CP-QPSK optical signal for three different cases:

- the Amplified Spontaneous Emission (ASE) source is lo ^¬ cated 350GHz away 44 from the CP-QPSK optical signal; - the Amplified Spontaneous Emission (ASE) source is lo ^¬ cated 850GHz away 45 from the CP-QPSK optical signal;

- the Amplified Spontaneous Emission (ASE) source is lo ^¬ cated 2.60THz away 46 from the CP-QPSK optical signal.

For this experiment, a wide-band, Amplified Spontaneous Emis- sion (ASE) source filtered by two cascaded 50 GHz optical band-pass filters has been co-propagated together with a 40G CP-QPSK signal over a 700 km fiber link, and the bit-error ratio of the data signal has been measured for different del ^¬ ta power values. As shown in Fig. 4, significant bit-error rate degradations of CP-QPSK signal over a large spectral re ^¬ gion could be observed when increasing the power of the ASE source. This is due to the fact that the Cross Polarization Modulation (XPolM) induced random polarization rotations cover all of the above-mentioned categories: slow, fast and ul- tra fast. In this case, the fast polarization rotations were responsible for the observed impairment.

Cross polarization modulation effects are expected to affect not only standard coherent receivers based on digital signal processing, but also direct-detection receivers employing fast polarization controllers for input polarization demultiplexing. Indeed, current active polarization controllers can compensate only relatively slow polarization rotations (in the order of a hundred kHz) but would not be able to cope with fast polarization rotations induced by Cross Polariza- tion Modulation (XPolM) . A large number o idler channels schemes, as well as transient suppression channels schemes based either on pola ^¬ rized/depolarized CW signals or on filtered ASE sources are known from the prior art. Cited, for example, is C. Headley, G. Agraval , "Raman Amplification in Fiber Optics Communica ^¬ tion Systems" Academic Press, December 30, 2004, or J. Ches- noy, G. Agrawal , I. P. Kaminow, and P. Kelley, "Undersea Fi ^¬ ber Communication Systems" Academic Press, October 3.

However, such conventional schemes have a severe impact on next generation transmission systems based on coherently- detected polarization-multiplexed optical signals such as CP- QPSK.

The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution that mi ^¬ nimize the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems such as CP-QPSK.

SUMMARY OF THE INVENTION

In order to overcome the above-described need in the art, the present invention discloses an optical communication system for transmitting an optical signal, comprising a pump source configured to generate a pump signal having rotating polari ^¬ zation, a polarization sensitive receiver for receiving the optical signal having a polarization tracking cut-off fre- quency, wherein the polarization of the pump signal is configured to rotate at a predetermined frequency of polarization rotation and the frequency of polarization rotation of the pump signal is higher than the polarization tracking cut-off frequency of the receiver. The polarization tracking cut-off frequency can be defined as a frequency of polarization variations that leads to factor of two higher BER at the polarization sensitive receiver comparing to case without.

In a next embodiment of the invention the optical signal is a coherently-detected polarization-multiplexed quadrature phase shift keying (CP-QPSK) signal.

It is also an embodiment, that the system further comprises a continuous wave (CW) laser source for generating a continuous wave (CW) laser signal. In other alternative embodiments of the present invention, the system further comprises a polarization modulator for modulating the continuous wave (CW) laser signal and a radio frequency (RF) source connected with the polarization modulator for generating the polarization rotation of the pump signal.

In a further embodiment, the system further comprises a first polarization rotator for rotating the polarization of the continuous wave (CW) laser signal and thereby generating a first component of the pump signal.

In a next embodiment, the system further comprises an optical phase modulator for phase-modulating the continuous wave (CW) laser signal and a radio frequency (RF) source connected with the phase modulator, the optical phase modulator being configured to generate a second component of the pump signal.

In an alternative embodiment, the pump signal is a combina ^¬ tion of the first component and the second component of the pump signal. It is also an embodiment, that the first component and the second component of the pump signal have a different polari ^¬ zation with respect to each other.

In other alternative embodiments of the present invention, the radio frequency (RF) source is configured to adjust the frequency of polarization rotation of the pump signal.

In a further embodiment, the system further comprises a first continuous wave (CW) laser source for generating a first continuous wave laser signal and a second continuous wave (CW) laser source for generating a second continuous wave (CW) laser signal, the first and the second continuous wave (CW) laser signals being frequency detuned with respect to each other.

In a next embodiment, the system further comprises a second polarization rotator coupled with the second continuous wave (CW) laser source for rotating the polarization of the second continuous wave (CW) laser signal and thereby generating a second laser signal.

In an alternative embodiment, the pump signal is a combina- tion of the first continuous wave (CW) laser signal and the second laser signal, the first continuous wave (CW) laser signal and the second laser signal having a different polari ^¬ zation with respect to each other.

It is also an embodiment, that the frequency of polarization rotation of the pump signal is adjustable by adjusting the frequency detuning of the first and the second continuous wave (CW) laser signals with respect to each other.

In a further embodiment, the apparatus further includes an optical fiber link for propagating the pump signal and the optical signal.

In an alternative embodiment, the polarization sensitive re ^¬ ceiver is a coherent receiver.

The problem stated above is also solved by a method for transmitting an optical signal which includes: generating a pump signal having rotating polarization, transmitting the pump signal, transmitting the optical signal, receiving the optical signal by means of a polarization sensitive receiver having a polarization tracking cut-off frequency, rotating the polarization of the pump signal at a predetermined fre ^¬ quency of polarization rotation, the frequency of polariza ^¬ tion rotation of the pump signal being higher than the polarization tracking cut-off frequency of the receiver. The method, the apparatus and the system provided, in partic ^¬ ular, bears the following advantages: a) They minimize the destructive Cross Polarization Modu ^¬ lation (XPolM) interactions in polarization-multiplexed transmission systems such as CP-QPSK. b) They allow the generic usage of optical pump sources independently on modulation format of data channels. c) They do not require necessarily the employment of high RF electronics. d) They are easy to implement. e) They provide a benefit related to power budget, mechan ^¬ ical footprint as well as cost efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained by way of example in more detail below with the aid of the attached drawings.

Fig. 1 is a schematic representation of the application of a conventional depolarization technique.

Fig. 2 is a schematic representation of the application of a conventional depolarization technique.

Fig. 3 is a representation 31 of the degradation of the Bit Error Ratio BER 32 of a 40G CP-QPSK optical signal due to a depolarized CW channel.

Fig. 4 is a representation 41 of the degradation of the Bit Error Ratio BER 42 of a 40G CP-QPSK optical signal due to an Amplified Spontaneous Emission (ASE) source.

Fig. 5 is a schematic representation of the application of a depolarization technique for application in polarization- multiplexed transmission system according to an embodiment of the invention. Fig. 7 is a schematic representation of the application of a depolarization technique for application in polarization- multiplexed transmission system according to an embodiment of the invention.

Fig. 8 is a representation 81 of the degradation of the Bit Error Ratio BER 82 of a 40G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources.

Fig. 9 is a representation 91 of the degradation of the Bit Error Ratio BER 92 of a 40G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources.

DESCRIPTION OF THE INVENTION

As regards the description of Figures 1 to 4, reference is made to the background of the invention.

Illustrative embodiments will now be described with reference to the accompanying drawings to disclose the teachings of the present invention. While the present invention is described herein with reference to illustrative embodiments for partic- ular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recog ^¬ nize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.

Fig. 5 is a schematic representation of the application of a depolarization technique for application in polarization- multiplexed transmission system according to an embodiment of the invention. In particular, Fig 5 shows a pump source 51, for example a Raman pump source, which includes a single CW light source 53, for example a laser source. The CW light source 53 generates a CW laser signal 56 which is modulated by a polarization modulator 55, in particular a fast polarization modulator, that generates the fast polarization rotation of the pump signal 52. The required speed of polari ^¬ zation rotations can be generated using the RF source 54, connected to the polarization modulator 55, which is configured to adjust the frequency of polarization rotation of the pump signal 52. The so generated pump signal 52 can be prop- agated in a fiber link together with an optical signal, for example a CP-QPSK signal. The optical signal and the pump signal 52 can be received by a polarization sensitive receiv ^¬ er having a given polarization tracking cut-off frequency. The ultra-fast polarization of the pump signal 52 can be ad ^¬ justed in such a way that the frequency of polarization rota ^¬ tion of the pump signal 52 is higher than the polarization tracking cut-off frequency of the receiver. As a consequence, the spectral components of the polarization rotation can be naturally rejected by the polarization sensitive receiver, which may also be a coherent receiver. The polarization tracking cut-off frequency can be defined as a frequency of polarization variations that leads to factor of two higher BER at the polarization sensitive receiver comparing to case without. In this way the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems can be efficiently minimized.

Fig. 6 is a schematic representation of the application of a depolarization technique for application in polarization- multiplexed transmission system according to an embodiment of the invention. In particular, Fig 6 shows a pump source 61, for example a Raman pump source, which generates a pump sig ^¬ nal 62, combination of a phase modulated component 68 and a phase un-modulated component 67, having different polariza ^¬ tion, derived from a single CW laser source 63. An optical coupler, for example a 3dB coupler (not shown in Figure 6) may split the single laser source 63 in two differently pola ^¬ rized CW signals 623 and 626. According to a different embo- diment of the invention, the two CW signals 623 and 626 may be orthogonally polarized. An optical phase modulator 66, phase-modulates the CW signals 626, while a polarization rotator 65, rotates the polarization of the CW signal 623 relative to the CW signal 626 . An optical coupler, for ex ^¬ ample a 3dB coupler (not shown in Figure 6) , may combine the two differently polarized CW signals 67 and 68. The required speed of polarization rotations can be generated using the RF source 64, connected to the optical phase modulator 66, which is configured to adjust the frequency of polarization rota ^¬ tion of the pump signal 62. The so generated pump signal 62 can be propagated in a fiber link together with an optical signal, for example a CP-QPSK signal. The optical signal and the pump signal 62 can be received by a polarization sensi ^¬ tive receiver having a polarization tracking cut-off frequency. The ultra-fast polarization of the pump signal 62 can be adjusted in such a way that the frequency of polarization ro- tation of the pump signal 62 is higher than the polarization tracking cut-off frequency of the receiver. As a consequence, the spectral components of the polarization rotation can be naturally rejected by the polarization sensitive receiver, which may also be a coherent receiver. In this way the de- structive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems can be effi ^¬ ciently minimized.

Fig. 7 is a schematic representation of the application of a depolarization technique for application in polarization- multiplexed transmission system according to an embodiment of the invention. In particular, Fig 7 shows a pump source 71, for example a Raman pump source, which generates pump signal 72, combination of two frequency detuned, differently pola- rized CW laser signals 75 and 78 from two CW laser sources 73 and 74. According to a different embodiment of the inven ^¬ tion, the two CW signals 75 and 78 may be orthogonally pola- rized. A polarization rotator 77, coupled with the second continuous wave (CW) laser source 74, which rotates the polarization of the continuous wave (CW) laser signal 76 rel ^¬ ative to the CW signal 75 thereby generating the CW signal 78. The fast polarization rotation is generated by combining the two laser signal 78 and 75. The frequency of polarization rotation of the pump signal 72 can be adjustable by adjusting the frequency detuning of the first 75 and the second 76 continuous wave (CW) laser signals with respect to each other. Hereto, speed of polarization rotation is directly related to the frequency detuning of the two CW channels.

The so generated pump signal 72 can be propagated in an fiber link together with an optical signal, for example a CP-QPSK signal. The optical signal and the pump signal 72 can be re- ceived by a polarization sensitive receiver having a polarization tracking cut-off frequency. The ultra-fast polariza ^¬ tion of the pump signal 72 can be adjusted in such a way that the frequency of polarization rotation of the pump signal 72 is higher than the polarization tracking cut-off frequency of the receiver. As a consequence, the spectral components of the polarization rotation can be naturally rejected by the polarization sensitive receiver, which may also be a coherent receiver. In this way the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems can be efficiently minimized.

The embodiment of the invention comprising the two detuned CW laser signal 75 and 78 does not require necessarily the em ^¬ ployment of high RF electronics; furthermore, it provides a benefit related to power budget, mechanical footprint as well as cost efficiency. Fig. 8 is a representation 81 of the degradation of the Bit Error Ratio BER 82 of a 40G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources.

In particular, Fig 8 shows the Bit Error Ratio BER 82 of a 40G CP-QPSK optical signal versus the frequency offset 83 be ^¬ tween the two orthogonally polarized, frequency detuned CW laser sources.

The 40G CP-QPSK receiver is characterized by a polarization tracking cut-off frequency of around 8 GHz.

For frequency offset below 10 GHz, significant degradations of data signal can be observed when co-propagating with the pump in a fiber link (optical power variation ΔΡ = 11.5 dB) . However once the frequency offset is larger than a critical minimum detuning (in the example shown in Fig. 8 ca. 12 GHz), the distorting influence of the high power pump signal is ef ^¬ fectively suppressed.

Fig. 9 is a representation 91 of the degradation of the Bit Error Ratio BER 92 of a 40G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources.

In particular, Fig 9 shows the Bit Error Ratio BER 92 of a 40G CP-QPSK optical signal versus the spacing 93 between the CW pump signal and the CP-QPSK signal.

Moreover, Fig. 9 shows that a frequency offset Δί of 15 GHz between the two CW signals leads to distortion-free perfor ^¬ mance almost independently of the spacing between the CW pump and the CP-QPSK channel.

The experimental analysis confirmed that a minimum frequency detuning (polarization rotation speed) is required in order to effectively suppress impairments on polarization multip ^¬ lexed data signals. Thus, controlled frequency spacing be ^¬ tween the two CW lasers may be needed.

The present invention allows for generic usage of optical pump sources independently on modulation format of data chan ^¬ nels.

For each modulation format and data rate, a critical minimum frequency detuning has to be identified individually. Theo ^¬ retically, a fix value for the maximum frequency detuning cannot be established. Larger frequency offset may be benefi ^¬ cial for reducing of pump beating, however this may reduce the optical bandwidth available for useful channels' trans ^¬ mission and may leads to higher sensitivity to frequency de ^¬ pendent polarization changes (due to second order polariza- tion mode dispersion, SOPMD or polarization dependent loss, PDL) .

As far as the CW sources are concerned, fixed laser with wa ^¬ velength locker or a tunable laser can be used. The two CW lights have to be combined at different polarizations (pre- ferably orthogonally polarized) . The power level of the las ^¬ ers should preferably be similar, but slight mismatches do not affect the performance significantly.

The present invention is not limited to the details of the above described principles. The scope of the invention is de- fined by the appended claims and all changes and modifica ^¬ tions as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. Mathe ^¬ matical conversions or equivalent calculations of the signal values based on the inventive method or the use of analogue signals instead of digital values are also incorporated. List of Abbreviations:

ASE Amplified Spontaneous Emission BER Bit Error Ratio

CW Continuous Wave

CP-QPSK Coherent Polarization-multiplexed Quadrature Phase

Shift Keying

DPSK Differential Phase Shift Keying DQPSK Differential Quadrature Phase Shift Keying

DWDM Dense Wavelength Division Multiplexing

EDFA Erbium Doped Fiber Amplifier

FWM Four Wave Mixing

ODB Optical Duo Binary OIF Optical Internetworking Forum

OOK On Off Keying

OTSC Optical Transient Suppression for C-band

OSA Optical Spectrum Analyzer

PBC Polarization Beam Combiner PBS Polarization Beam Splitter

PDL Polarization Dependent Loss

SOPMD Second Order Polarization Mode Dispersion VOA Variable Optical Attenuator

XPolM Cross Polarization Modulation